This invention relates to the art of correctors for use in common amplification transmitters wherein aural and visual signals are commonly amplified.
It is known to provide common amplification in television transmitters wherein the visual and aural carriers are commonly amplified. The U.S. Pat. to Ta, et al. No. 5,198,904 noted that a problem resulting from such common amplification is aural carrier distortion. The distortion occurs in both the phase and amplitude domains of the aural carrier. This is sometimes referred to as xe2x80x9cvision to aural crosstalkxe2x80x9d distortion and is caused by the transfer of modulation from a vision carrier to an aural carrier in a combined amplifier. This is primarily due to the non-linear characteristic of the power amplifier used in the transmitter.
The patent to Ta proposes to correct the unwanted distortion of the aural carrier by predistorting phase and amplitude components of the aural carrier so as to be directly opposite to the unwanted distortions caused by the common amplification in a television transmitter. To this end, Ta samples the baseband video signal prior to the signal being used to modulate a visual IF carrier signal. The sampled baseband video signal is delayed by a video delay to compensate for the delays that are introduced in an IF vision modulator to which the baseband video signal is also supplied for modulating a visual carrier signal. The delayed baseband video signal is then applied to a complementary nonlinear amplifier which provides a phase correction signal and an amplitude correction signal. These correction signals are supplied to an amplitude and phase modulator which also receives the aural carrier. The aural carrier is modulated by the amplitude and phase correction signals to provide a modified aural signal which is summed with the output of the IF vision modulator.
The U.S. Pat. to D. Culling No. 5,418,578 is also directed to correcting an aural carrier signal in a common amplifier system. The Culling patent discloses a system wherein the correcting serves to minimize cross-modulation distortion of the aural carrier signal caused by common amplification of aural and visual frequency signals. The correcting apparatus in Culling provides circuitry for receiving a modulated visual signal comprised of a visual carrier signal modulated by a video baseband signal and providing therefrom a detected video signal. The detected video signal is received by an aural corrector which provides therefrom a phase correction signal and an amplitude correction signal. These signals are supplied to phase and amplitude modulation circuitry for modulating an aural carrier with the phase and amplitude correction signals to provide a corrected aural signal which is opposite the cross-modulation distortion. The visual signal and the corrected aural signal are then combined.
It is to be noted that the Ta patent and the Culling patent provide both a phase correction signal and an amplitude correction signal which are then used to phase and amplitude modulate the aural carrier to provide a corrected aural carrier.
Another aural correction system is described in a paper entitled xe2x80x9cAural/Visual Multiplex Operation of Klystron Type UHF Television Transmittersxe2x80x9d, The Pennsylvania State University Graduate School, report in Engineering Science by Ronald W. Zborowski, March 1981. The Zborowski paper discloses (in FIG. 12) an aural corrector which performs phase correction to eliminate phase modulation of an aural carrier but does not perform amplitude correction and does not include an amplitude modulator for amplitude modulating the aural signal with an amplitude correction signal as in the Ta and Culling patents noted above. Also, the Zborowski paper does not provide a second aural corrector for receiving a modulated visual carrier signal and a phase corrected aural carrier signal to provide therefrom a combined corrected aural carrier signal which is then combined with the visual carrier signal to provide an output signal.
It is an object of the present invention to provide an aural correction system which does not employ generation of an amplitude correction signal and does not employ an amplitude modulator for modulating the aural carrier with the amplitude correction signal.
In accordance with one aspect of the present invention, apparatus is provided for correcting an aural carrier signal in a common amplifier system wherein the correcting serves to reduce cross-modulation distortion of the aural carrier signal caused by non-linearities of the common amplifier system. An aural corrector receives a modulated visual carrier signal comprised of a visual carrier signal modulated by a video baseband signal and a phase corrected aural carrier signal and provides therefrom a combined corrected aural carrier signal. The modulated visual carrier signal is combined with the combined corrected aural carrier signal in such a manner to provide a corrected output signal.
Another aspect of the present invention provides an aural corrector for reducing or removing vision to aural crosstalk in a combined amplifier, which corrector includes apparatus for providing an original IF vision signal and an IF aural signal. The IF aural signal and an IF vision signal in phase with the original IF vision signal are combined. Also, the IF aural signal and an IF vision signal which is out of phase with the original IF vision signal are combined. A first non-linear circuit introduces non-linear characteristics to the combined in phase IF vision signal and IF aural signal such that the non-linear characteristics of the combined in phase IF vision signal and IF aural signal are the inverse of the non-linear characteristics of the combined vision and aural signal at the output of the amplifier. A second non-linear circuit introduces non-linear characteristics to the combined out of phase IF vision signal and IF aural signal such that the non-linear characteristics of the combined out of phase IF vision signal and IF aural signal are the inverse of the non-linear characteristics of the combined vision and aural signal at the output of the amplifier. A combiner serves to combine the corrected combined in phase IF vision signal and IF aural signal with the corrected combined out of phase IF vision signal and IF aural signal to produce a corrected IF aural signal such that when the original IF vision signal and the corrected IF aural signal are commonly amplified, any vision signal modulated onto the aural signal is substantially reduced.
In accordance with another aspect of the present invention, the aural correcting apparatus includes circuitry for receiving a modulated visual carrier signal comprised of a visual carrier signal modulated by a video baseband signal and providing therefrom a sampled video signal. A first aural corrector receives the sampled video signal and provides therefrom a phase correction signal. A phase modulator receives the aural carrier signal and this signal is modulated with the phase correction signal to provide a phase corrected aural carrier signal. The second aural corrector receives the modulated visual carrier signal and the phase corrected aural carrier signal and provides therefrom a combined corrected aural carrier signal. The visual carrier signal and the combined corrected aural carrier signal are then combined to provide an output signal.